Group B Streptococcus (GBS) is the leading cause of neonatal sepsis and meningitis. Neonatal GBS diseases occur in two forms: early-onset and late-onset. Early-onset neonatal GBS disease occurs within 7 days of life, and the late-onset disease occurs after 7 days of life. Prevention of neonatal GBS through vaccination can only be achieved through maternal transfer of antibodies to newborn infants. GBS capsular polysaccharide (Ia, Ib, II, III, IV, V) is a virulence factor and protective antigen. Vaccines based on capsular polysaccharide and protein conjugation are being developed at NICHD as well as other laboratories. Determination of a protective serum antibody threshold would provide an important target for which candidate GBS vaccines administered during pregnancy should aim. The major objective of the study is to evaluate, for each GBS serotype, the pattern of the protective association between naturally acquired, serotype-homologous maternal serum IgG antibody titers and the risk of development of early-onset GBS disease. The surveillance began as of July 1995. The target is to accrue 140 early-onset GBS cases and 1120 controls (colonized infants) during the 30 month study.